The advantages of using impulse or short pulse radar for detecting discontinuities in dielectric media is well recognized as disclosed in U.S. Pat. No. 4,072,942 to Alongi, U.S. Pat. No. 4,008,469 to Chapman and U.S. Pat. No. 3,806,795 to Morey. Systems of the type disclosed in these patents operate by radiating a pulse of only one or a few excursions containing a broad spectrum of radio frequencies enabling the system to detect target phenomena of widely varying characteristics. When radiated into a dielectric medium, short pulses are reflected by discontinuities in the medium in a manner which allows the pulse echo to be detected and analyzed to provide information about the location and size of the discontinuity. Generation of a short pulse in real time, however, involves two serious drawbacks. The first is rne necessity of recording the high-frequency data with a reasonable dynamic range, while the second is the problem of designing an antenna capable of coupling the broad band energy efficiently into the ground. The first problem can be solved in part by using a heterodyne receiver technique. The problem of efficient antenna coupling is much more difficult. Normally, attempts to solve this problem have involved building a broad band antenna designed to have a minimum of reflections. While such an antenna operates well to transmit energy into the medium, it is by necessity a low-gain antenna system.
One prior attempt to overcome the drawbacks of real time pulse radar has been the development of a synthetic pulse radar in which continuous wave measurements are made at many selected frequencies defining a Fourier spectrum of frequencies equivalent to the bandwidth of a short radio frequency pulse. This prior art system is disclosed in Robinson, L.A. et al, "Location and Recognition of Discontinuities in Dielectric Media Using Synthetic RF Pulses," Proceedings of the IEEE, Vol. 62, No. 1, January, 1974, pages 36-44, and in Robinson, L.A. et al, "An RF Time Domain Reflectometer Not in Real Time", IEEE Transactions on Microwave Theory and Tech., Vol. MTT-20, pages 855-857. In the system disclosed in these articles, a computer is used to control the sequence of measurements, to store the measured parameters and to process the stored parameters to permit a synthetic pulse echo to be displayed. Since the amplitudes and phases of the spectral lines can be individually controlled, the synthetic radar pulse may be shaped to achieve optimum tradeoff between short pulse width, small ringing on the baseline between pulses, and total bandwidth covered by the spectrum.
The Robinson et al system included a tunable oscillator requiring external frequency-stabilizing circuitry to obtain the necessary degree of stability in the frequency output of the oscillator. Such stabilizing circuitry adds to the expense and complexity of the synthetic pulse system, and a tunable oscillator is not compatible with standard digital logic circuitry, such as a microprocessor control circuit. Consequently, a digitally controlled, synthetic pulse radar apparatus was developed which was more compatible with integrated circuit control components. This apparatus, disclosed in U.S. Pat. No. 4,218,678 to Fowler et al includes a master oscillator for generating a base periodic signal which is provided to a synthesizer in a transmitter. The synthesizer generates a Fourier spectrum of frequencies for a desired synthetic radar pulse by successively multiplying the base periodic signal by each integer in a series of integers represented by digital signals received from a microprocessor based controller. The transmitter also includes an attenuator control for controlling the strength of the periodic component signals making up the Fourier spectrum in response to an attenuator control signal from the microprocessor based controller. The output from the attenuator control is suitably amplified and transmitted by a transmitter antenna.
The Fowler et al system includes a receiver designed to recover representative parameters of each of the periodic component signals which have been broadcast by the transmitter and modulated by the geophysical phenomena being measured. The receiver includes a frequency synthesizer similar to that used in the transmitter which multiplies the base periodic signal from the oscillator by each integer received from the microprocessor based controller. The recevier also includes a quadrature circuit which receives the signals from the frequency synthesizer and produces both in-phase and quadrature reference signals having a known fixed frequency to the broadcast periodic component signals. The quadrature and in-phase reference signals have a 90.degree. phase difference relative to each other. Both the in-phase reference signals and the quadrature reference signals are then mixed with the electrical signal representative of the portion of the periodic component signal returned from the geophysical phenomena to yield both phase and amplitude information.
The synthetic pulse radar system disclosed by the Fowler et al patent eliminates many of the impractical complexities of prior art systems and is a distinct improvement over such systems. However, even this improved system provides for the transmission of only a single frequency at a time, and the single transmitted frequency must be accurately reproduced in the receiver. This limits to some extent the sensitivity of the system. Furthermore, the transmission of only a single frequency during a geophysical survey results in a relatively slow data acquisition rate which increases the time required for the survey.